Viscosity - Blood Flow

The resistance to flow in a tube results from inner friction in the fluid i.e. the viscosity. We all know that water and sugar syrup do not flow at the same rate from a bottle. We can say that water has low viscosity and syrup, a high viscosity. For convenience viscosity of a fluid is expressed relative to the viscosity of water. Blood plasma has a relative viscosity of 1.8 mostly as a result of the 7% dissolved proteins. Whole blood is more viscous because of the cells in it, at 37?C, relative viscosity of mammalian blood is between 3 and 4. Therefore, because of the presence of RBC blood behaves as though it is 3-4 times more viscous than water. However, blood does not behave as expected of a viscous fluid.

Its relative viscosity changes with decreasing radius of the blood vessels. In fact in tubes less than 0.3 mm in diameter the relative viscosity of blood approaches that of the plasma, therefore, it flows more easily. In flowing blood, we find that the red cells tend to accumulate in the centre. This accumulation leaves the wall relatively free of cells, therefore, the viscosity in the centre is more than at the sides. Since flow is inversely related to viscosity, flow at the walls will increase slightly and will decrease at the centre slightly. Another peculiar aspect of blood flow in capillaries is that often the capillary diameter is smaller than RBC and the RBCs easily change shape to pass through the capillary. This gives rise to a very different type of flow - bolus flow in which the red cells act as a plug that causes rapid increase in liquid along the walls of the capillary and thus help in the renewal of the diffusible substances in this layer.